1. Field of the Invention
The present invention relates to a photomask, and more particularly, to a photomask that solves a problem related to charging in a focused ion beam (FIB) system, a method for manufacturing the same, and a method for detecting/repairing defects in a photomask.
2. Description of the Related Art
A photolithography process for forming microcircuits on a wafer is an important aspect of processes for manufacturing a semiconductor device. A photomask is a prototype of a circuit used in the photolithography process. Patterns formed on the photomask are transferred by radiation such as light onto a wafer through a reduction lens in the photolithography process. As a result, defects in a photomask cause defects in wafer manufacturing processes and result in defective chips. Thus, developing a technology for manufacturing a photomask without defects or for precisely detecting/repairing defects in a photomask has become important.
A scanning electron microscope (SEM) may be used to detect defects in a photomask. In a SEM, electrons with high energy are transmitted to a sample of a photomask. Since some of the incident electrons are transmitted to the sample and reflected at a boundary between the sample and an underlying layer of the sample, images of photomask patterns detected by the SEM become larger than images of a desired portion. Also, a mask substrate may be damaged by the transmission of electrons.
Thus, a focused ion beam (FIB) system, in which images of photomask patterns are obtained by detecting secondary electrons, which are generated by the collision of ions accelerated by a high voltage with the sample of a photomask, has been used as a means for detecting defects in a photomask. Further, both opaque defects and clear defects in a photomask are removed using the FIB system.
The step in which positive xe2x80x9c+xe2x80x9d ions (usually, positive gallium Ga+ ions) transmitted to the sample from the FIB system affect a conductive pattern of a photomask will now be described with reference to FIG. 1. In general, a substrate 10 of a photomask is formed of a transparent nonconductor such as quartz, and opaque patterns 12a, 12b, and 12c formed of conductive materials are separated from one another. Chromium (Cr) or chrome is typically used as a conductive material.
The conductive pattern 12b becomes positively charged by the Ga+ ions. However, the positive electric charges cannot be dispersed to the adjacent conductive patterns 12a and 12c because the substrate 10 is an nonconductor.
Electrons (secondary electrons) are emitted from the conductive pattern 12b when high energy GA+ ions collide with the conductive pattern 12b. The secondary electrons are combined with the transmitted GA+ ions, which means that the secondary electrons are trapped by the transmitted GA+ ions, and thus, the number of secondary electrons detected by the FIB system becomes smaller, and the contrast of images of the photomask patterns is reduced.
Furthermore, as semiconductor devices become highly integrated, the size of circuit patterns is reduced, and thus the size of the conductive patterns 12a, 12b, and 12c of the photomask becomes smaller. As a result, the GA+ ions are rapidly charged, and thus more secondary electrons are trapped.
FIG. 2 illustrates images taken by a focused ion beam (FIB) system of a photomask when a plurality of chrome (Cr) patterns are formed on a quartz substrate. The images are not clearly classified into a portion formed of chrome (Cr) and a portion formed of quartz. That is, the contrast of the images of photomask patterns is not high.
Defects in images of photomask patterns caused by the trapping of the secondary electrons are inevitable even in the FIB system. Thus, errors occur when precisely detecting defects in a photomask and removing them. Finally, defects occur in patterns on a wafer.
To solve the above problem, it is a first objective of the present invention to provide a photomask and a method of manufacturing the photomask such that images of photomask patterns having high contrast can be obtained when using a focused ion beam (FIB) system.
It is a second objective of the present invention to provide a method for precisely detecting and repairing defects in a photomask.
According to one aspect of the present invention, there is provided a photomask for a focused ion beam (FIB) system. The photomask includes a mask substrate formed of a transparent nonconductor, a plurality of opaque conductive patterns formed on the mask substrate and separated from one another, and one or more conductive lines for connecting one of the conductive patterns with at least one adjacent conductive patterns.
The charges of positive xe2x80x9c+xe2x80x9d ions applied to conductive patterns are dispersed through conductive lines, thus increasing the amount of secondary electrons emitted from the conductive patterns.
It is preferable that the conductive lines be formed of gallium (Ga) containing carbon (C), and the conductive lines have a thickness and a width that allows incident light to be transmitted. The conductive lines have a thickness and a width that allows incident light to be transmitted. The width of the conductive lines is less than half the wavelength of an ion source used in the FIB system, and the height of the conductive lines is less than a skin depth. The conductive patterns are formed of chromium or chrome (Cr).
According to another aspect of the present invention, there is provided a method for manufacturing a photomask. A mask substrate formed of a transparent nonconductor is prepared. A plurality of opaque conductive patterns separated from one another are formed on the mask substrate. One or more conductive lines for connecting one of the conductive patterns with at least one adjacent conductive patterns are formed on the mask substrate.
The conductive lines are formed using a FIB system, and the conductive lines are formed of gallium (Ga) containing carbon (C).
In accordance with another aspect of the invention, there is provided a method for repairing defects in a photomask. Ions emitted from a focused ion beam (FIB) system are collided with conductive patterns of the photomask including a mask substrate formed of a transparent nonconductor, a plurality of opaque conductive patterns formed on the mask substrate and separated from one another, and one or more conductive lines for connecting one of the conductive patterns with at least one adjacent conductive patterns. Electrons emitted from the conductive patterns of the photomask are detected. Images of the conductive patterns of the photomask are obtained using the detected electrons. It is determined from the images of the conductive patterns whether defects in the photomask exist. That is, it is checked whether there is a portion where chrome (Cr) is not deposited, of the conductive patterns (clear defects), and whether chrome (Cr) is formed at a portion where a quartz substrate should be exposed (opaque defects). If defects in the photomask exist, the defects in the photomask are removed by using the FIB system. The conductive lines are simultaneously removed when the defects in the photomask are opaque defects and the opaque defects are removed. On the other hand, the conductive lines are removed using a laser.